Green

Good colour, green.

While doing a picture today, I found that I needed more greens. Some of the leaves were almost yellow, while in the darkest recesses the shadow greens were so dark, intermingled with purples.
Colour is a strange phenomenon actually. Did you know that the brain can't remember colour?

Or that the eye can't see as many variations in yellow as it can in all the other colours?

Colour is measured nowadays on quite sophisticated spectrophometers. These machines can fairly accurately match colours, suggesting what colour needs adding to achieve the required shade. They are still not as good as the experience of a good technician. I reckon that our team can all beat the Spectro every time. It will get the right result, but not as quickly as we humans.

The measurement of colour is based on a mathematical system, in which colour is represented by a three dimensional sphere.

The system is known as CieLab. The L axis measures lightness/darkness and goes from white to black. The a axis measures the red/green spectrum, while the b axis measures Yellow/blue.using those three parameters it's possible to measure colour.
The overall colour measurement is known as deltaE. (no mathematical symbols on the iPad so I ned to type the full words).

The one other influence on a colour is chroma. Chroma is the strength of a colour, which is difficult to explain, but is essentially how much colour is available to be reflected in the visible light spectrum.
So we have darkness/lightness; chroma and HUE, which is the colour itself.

I find it a fascinating subject, colour chemistry. But I do realise that I have probably bored you to death by now so I'll shut up and get back to choosing the right green for my picture......